A composite reverse osmosis membrane is preferably used for many purposes such as production of ultrapure water and desalination of sea water or brackish water. This membrane also can be used to remove and withdraw contaminants which are the cause of pollution, for example, waste materials from dyeing baths or electrochemical deposition coating systems. The contaminated water can be cleaned for reuse. Furthermore, it is possible to condense effective components for food and to remove harmful matters for purified water and sewage.
Conventionally, a composite reverse osmosis membrane, which is obtained by forming a skin layer substantially having selective separating properties on a microporous support, has been known as a reverse osmosis membrane whose structure is different from that of an asymmetric reverse osmosis membrane.
There have been composite reverse osmosis membranes in which a skin layer comprising polyamide obtained by interfacial polycondensation of polyfunctional aromatic amine and polyfunctional aromatic acid halide is formed on a support (for example, Japanese Laid-Open Patent Publication Nos. 55-147106, 62-121603 and 63-218208, Japanese Patent Publication No. 6-73617, and the like).
The composite reverse osmosis membranes described above have a high desalination performance and a high flux performance. However, it has been desired that the composite reverse osmosis membranes should further improve the flux performance while keeping a high desalination performance with respect to a reduction in operation and equipment costs, efficiency and the like. For these requirements, various additives have been proposed (for example, Japanese Laid-Open Patent Publication No. 63-12310), which have improved performance but are insufficient. Furthermore, after-treatment of the composite reverse osmosis membrane performed by using chlorine and the like also has been proposed (for example, Japanese Laid-Open Patent Publication Nos. 63-54905 and 1-180208).
However, all of the prior arts described above improve the flux performance of the membranes by utilizing certain types of deteriorating reactions for the polymers. Therefore, while ionic substances such as sodium chloride can be removed by charged membranes, inorganic or organic substances having low or no charge may permeate the membranes through defects at the molecular level where polymer chains are cut due to the deteriorating reaction, thereby reducing the rejection of these substances. For example, the rejection of isopropyl alcohol, which is used as a measure of rejection, is greatly reduced due to the deteriorating reaction caused by after-treatment as described above, so that it cannot be used in producing ultra pure water. Furthermore, with regard to organic harmful matters in drinking water recently at issue, the rejection is also reduced in the same way as described above, so that the required water quality cannot be obtained.